Imagine a music app with instant access to any song you wanted to hear. Imagine creating a playlist from those songs and quickly, easily sharing it with friends. Such an app does exist, it's called Spotify, and it could change music forever.

Hyperbole alert! I admit it, I'm over-the-moon about Spotify—both over what it currently is and more importantly over the potential it has. If you get a chance to try it out, I think you may feel the same way.

(Click any of the full-width images for a closer look.)

What Is Spotify

In short, here's how it works: Spotify is a peer-to-peer music streaming service; it's a desktop application, but its content all comes via the cloud. Think of it as though the entire iTunes Music Store were actually just your library, and that instead of the poorly designed mess that it is, imagine that it was refreshingly streamlined, fast, and easy to search and use. That gives you a little bit of an idea what Spotify is like. It also works under Windows, Mac OS X, and Linux (using Wine).

The best part: It's completely legal. Spotify seeks out licensing deals with the music industry before going live, meaning it sits comfortably in the 100% legit territory.

The catch: Oh, you knew there'd be a catch, didn't you. Although they've told me they hope to make it stateside before the end of the year, Spotify is currently only available in Sweden, Norway, Finland, the United Kingdom, France, and Spain.

But after just one day of playing with Spotify, I was convinced it could change the way I listen to music. After a week, I'm absolutely hooked.

What's So Great About Spotify?

First, just because it's really well done, let's take a look at the official Spotify commercial:

The best part about Spotify is that it could not be easier to use. Currently Spotify boasts around 3.8 million tracks (not as impressive as iTunes' over 10 million songs by any stretch, but they're adding songs regularly and could easily someday match iTunes). Here's a quick overview of the best features you'll notice as soon as you fire up Spotify for the first time:

You can access your music from any computer: Each time you open Spotify, you log into you Spotify account—meaning that no matter where you're logging in from, all of your playlists are ready and waiting for you. It's not synchronization—it's better than that. That's because you don't have to worry about moving gigabyte after gigabyte of music from one computer to the next. Spotify simply streams the music to wherever, and so logging into the application is essentially like logging into your Gmail account—except instead of all those emails, you've got all your music ready and waiting to play.

Sharing any of your playlists is as easy as sharing a link: iTunes and other desktop players do a fair job of playlist creation, but if you want to share a playlist with a friend, your best bet is still to burn the mix to a CD. Yeah... old school, right? We've featured plenty of web-based music players designed to make sharing playlists a breeze, including MixTape.me (particularly notable to me because I created it).

With Spotify, sharing a playlist you've created is as simple as right-clicking any playlist in the sidebar and selecting Copy HTTP link or Copy Spotify URI. Send either link to a friend via email and clicking on it should send them straight to that playlist in Spotify. It's dead simple. A web site has already sprung up dedicated to sharing Spotify playlists. On top of that, Spotify already has tools built in for quickly sharing a playlist via Facebook or Delicious. Pretty cool stuff.

Collaborating on playlists is a breeze: But let's say you're not impressed with the simple playlist sharing. Spotify also lets you turn any playlist into a collaborative playlist by, again, simply right-clicking the playlist and then clicking Collaborative Playlist.

It's clean and easy to navigate: Spotify's navigation is a little like a hybrid between traditional music players and the web. (Don't let that confuse you, it's not like Songbird.) All you get when you start is a search box, a home page, some Spotify radio stations, and a play queue.

The first time you search, you'll notice that Spotify saves your most recent searches in the sidebar. As soon as you start creating playlists, they show up below your saved searches. Click on any artist to go straight to every song by that artist in Spotify, along with their top hits, a biography section, and a radio station for that artist. Likewise, clicking on any individual album will take you to an album overview page containing all the tracks for that album and an album review. You can navigate backward and forward through your history with the forward/back buttons in the top left of the app—just like in your web browser.

But What About Quality?

In Spotify's free version, your music streams at approximately 160kbps using the Ogg Vorbis q5 codec (an open-source codec). Premium subscribers (that is, folks who pay 10 Euros a month, or around $14) stream at a higher bit rate of 320kbps (among other benefits).

Wait... I Have to Pay for It?

No, you don't have to pay for it, but yes, there are different versions of Spotify. The free version is supported by ads, but apart from those ads, supports all the same search, radio, and sharing features I mentioned above.

Spotify ads are actually audio ads occasionally inserted into your playlists. I haven't actually heard one in the time I've tested Spotify, but presumably they're there, and yes, one might imagine that such ads could get a bit annoying. (One user described Spotify ads thusly: "It was streets ahead of most desperate, shouty radio ads: polite, informative and reasonably unobtrusive. The ad for Watchmen that turned up a dozen songs later was a bit more traditional "In a world…" movie trailer voiceover fare.") Of course, that's where the Premium subscription—or even the $1 day pass if you just want a breather for a day—comes in.

What's Not So Great About Spotify

Now that I've hyped it up, it's time to acknowledge where Spotify isn't quite ready to replace your traditional desktop music player (like iTunes).

First, these songs are all trapped in the cloud: So how does one go about playing music when no internet connection is available? Actually, Spotify automatically saves some of the music to your hard drive as it plays (you can adjust just how much in the Preferences), so you're not entirely out of luck. Still, those 3.8 million songs shrink pretty quickly when you can only cache a portion of the music.

You can't sync the music to your MP3 player: If you want to Spotify music with you on your MP3 player, you really can't, right? Well, actually, that's partly—but not entirely—true. Spotify will probably never work on your old MP3 player, but if your player has Wi-Fi access (or, better yet, 3G access), that's another story. In fact, Spotify already has submitted an iPhone app to Apple, which you can see in the video below:

Let's pretend for a second that Apple would ever consider approving Spotify, despite their lame, indefensible app approval habits. Think about how convenient Spotify on the iPhone would be: You would never have to plug in your iPhone to sync your music. Just fire up Spotify and stream any of your playlists and tunes using your iPhone's data plan or Wi-Fi connection. It even supports offline playback the same way the desktop version does. (Okay, so we cheated on that one. It's not really that much of a bad thing—in fact, it's exactly the kind of easy "syncing" we'd like on an iPhone, Android phone, Pre, or any other cellphone.)

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If a song isn't in the Spotify catalog, you're out of luck: Spotify has a finite catalog, and unfortunately—for the time being, at least—you can't add anything of your own to that catalog. (I was knocked on my face when I wanted to play some ELO and realized they didn't have Flashback in their catalog, for example. In fact, it actually demonstrates another Spotify annoyance of mine: Artist pages are often filled with kind of lame compilations; it'd be nice if you could filter artist releases and compilations.)

This, to me, the inability to add your own music is clearly the biggest limitation of Spotify's potential. Even if the service managed to support every major and even relatively minor label in the world, there'd still be those tracks—like from your friend's band—that will likely never make their way to Spotify. Naturally, the folks at Spotify are aware of this problem, and they've even said they're considering some form of personal uploading, but right now it's not available, and implementing it is likely problematic for their licensing.

It's not available in my country yet: Okay, so I said the library limitation above is the biggest problem with Spotify. To be fair, that's the biggest problem for people who can actually use the application. For the whole of the United States and several other countries, Spotify's biggest problem is that it's not even available. As I said above, Spotify is working to change that; they're already in talks with the major U.S. labels (Wired quotes founder Daniel Eks as saying that "the labels want to see Spotify in the U.S."), so it's likely just a matter of time.

Your Thoughts

I realize this review is awfully glowing, but I also know that some of the problems with Spotify I've mentioned above are absolute showstoppers for some. In fact, some folks, the digital packrats—likely the people who have a hard time getting rid of their old CD collection despite having burned it all to their computer—may have no desire to ditch their hard drives full of music for a cloud-based alternative. We respect that.

Whether or not you've had the chance to play around with Spotify, love it or lump it, let's hear what you think about the app in the comments.